In known pulverizing machines the classifier means comprises a rotary classifier, and the machines are arranged so that a flow of air and particulate material is conveyed from the reducing zone to the rotary classifier, from which rotary classifier oversize particles of the material are returned to the rotor for further reduction.
In some of these pulverizing machines, for example the machines described in British patent specification No: 1333044, the pulverizer rotor and the housing provide spaced apart surfaces between which surfaces the air and particulate material flow passes in a direction towards the classifier and the oversize particles pass through this flow against the direction of flow, which gives rise to certain disadvantages where considerable quantities of oversize particles are being returned due to the latter impeding the flow. For example, the machines need a very considerable air supply to maintain the flow, and have a consequential high power consumption.
These disadvantages are reduced in other known forms of the pulverizing machines, which machines are arranged to provide a return path for the oversize particles to return to the rotor without passing completely through said flow to the classifier. However, the known return paths return the oversize particles to mix with the un-reduced material fed to the rotor by a feed means so that the returned oversize particles undergo the substantially same reduction processes as the un-reduced material, for example, as indicated in FIGS. 8-50 of the "Chemical Engineers Handbook" published in 1973 in the U.S.A. by McGraw-Hill Inc. Such pulverizing machines thus produce large amounts of particles which are reduced to a much smaller size than the maximum size acceptable, i.e. excessive reduction arises, with a consequential heavy power consumption.
In other known forms, for example, the Mikro-ACM Pulverizer shown in FIGS. 8-51 of the "Chemical Engineers Handbook" published in 1973 in the U.S.A. by McGraw-Hill Inc., a shroud is provided between part of the flow path to the rotary classifier and the return path. However, baffles are required to enable the air flow to move the particulate material inwardly toward the axis of the rotary classifier, for classification. Baffles are located in said part of the flow path to ensure that the flow has little or no rotational momentum. Consequently, substantially the whole of the material in the flow must enter and be accelerated rotationally by the classifier and the oversize particles ejected from the classifier against the flow direction, if the passage of unclassified material to the return path is to be prevented. This arrangement gives rise to other disadvantages. For example it imposes considerable demands on the design, operation and power supplies of the classifier, with a consequential heavy power consumption.
An object of the invention is to enable the power consumption to be reduced or utilized more efficiently while enabling the aforementioned disadvantages to be avoided or reduced.